1. Field of the Invention
The present invention relates to a technical field of a screw compressor.
2. Description of the Related Art
A conventional screw compressor heretofore known, as shown in FIG. 6, has the constitution in which a male rotor and a motor out of a pair of female and male screw rotors have a mutual shaft in common. In this case, since a coupling for connecting a shaft of the screw rotor and a shaft of the motor is unnecessary, it is economically advantageous. However, despite that the weight of a rotor of the motor is heavy, a rotor shaft for supporting the rotor is supported by way of only the screw rotor side, and therefore, the critical speed (rotational speed when the bending natural frequency in the flexing direction of a rotational shaft coincides with the rotational frequency) of the screw rotor is low. This screw compressor is applied to an oil cooling type screw compressor. In that case, since the compressor is operated at the low speed rotation of about 3600 rpm, it can be always operated at a rotational speed less than the critical speed, the possibility posing a problem being low.
Incidentally, a consideration is made that the screw compressor is applied to an oil free screw compressor. Then, the possibility of operation at the high speed rotation about 20000 rpm, that is, the rotational speed in excess of the critical speed, increases. That is, there is the time exceeding the critical speed. Then, as shown in FIG. 7 of an explanatory view of a relationship between the rotational frequency when passing through the critical speed (at the start time) and the amplitude, the amplitude of the rotor shaft increases directly after exceeding the critical speed, increasing the possibility that an inconvenience occurs in contact of screw rotors. Therefore, it has been difficult to employ the conventional screw compressor for the oil free screw compressor.
Incidentally, there is contemplated the measures that as shown in FIG. 7, since the amplitude of the rotor shaft gradually becomes small as assuming the higher speed rotation than the critical speed, the speed increasing acceleration of the rotational frequency of the rotor is increased when exceeding the critical speed to allow the rotational frequency at which the amplitude of the rotor shaft increases to pass through in a short period of time. However, this has a limit naturally, and is not always practical.
It is an object of the present invention to provide a screw compressor capable of being operated at a speed less than the critical speed while raising the critical speed than that of prior art. A further object of the present invention is to provide a screw compressor capable of being applied to an oil free screw compressor, without occurrence of inconvenience of the contact of screw rotors.
The screw compressor according to the present invention comprises a compressor body comprising a pair of female and male screw rotors meshed with each other, a rotor casing for encasing the screw rotors, a motor for driving one out of the screw rotors, and a motor casing for encasing the motor and connected integral with the rotor casing (of the compressor body). One out of the screw rotors and the motor have a shaft in common. Opposite ends of the shaft are supported by bearings. A portion between the motor and the screw rotor is supported by an intermediate bearing.
Preferably, a seal part is peripherally provided between a bearing located on the end side of the shaft on the motor side and the motor, and between the motor and the screw rotor.
Further, preferably, compression gas of higher pressure than that acting on the seal part between the motor and the screw rotor is introduced into the motor casing.
Further, a bearing for supporting opposite ends of the shaft and the intermediate bearing may comprise a rolling bearing.
Further, a motor may be disposed on the suction side of the screw rotor, and a pressure receiving portion at which generates the force in a direction from the motor toward the screw rotor may be provided on the end on the motor side of the shaft.
Further, the pressure receiving portion may comprise a piston of a cylinder portion.
Furthermore, a pressure receiving portion may be provided in close contact with a piston member provided, in the periphery of an outer race of a bearing, integral with the outer race and capable of being moved forward and backward in the longitudinal direction of an output shaft, and the piston member and a screw rotor with respect to the piston member may comprise a diaphragm for partitioning a space portion on the opposite side.